Proterozoic Belt Supergroup rocks east of Missoula, MT

The Belt Basin stretches from Eastern Washington to central Montana and north into Alberta.  The "Belt" is about 1.5 kilometers thick around Missoula and up to 5.5 kilometers thick around Helena and Butte.  For information on the Belt I recommend speaking with Professor Winston at the University of Montana  as he has been working on it for a few decades.